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- 7. Antennas as Part of All Communications Systems
- 21. Omnidirectional Radiation Pattern Horizontal Pattern Vertical Pattern
- 22. Unidirectional Radiation Pattern Horizontal Pattern Vertical Pattern
- 27. Antenna Types
- 28. Radiation Pattern of Half-wave Dipole Antenna 3-D view Vertical section Horizontal section
- 29. Omnidirectional Antennas Omnidirectional antenna Hertz antenna 1
- 30. Marconi Antenna
- 32. Radiating Coaxial Cable Antenna Radiating Coaxial Cable Antenna Radiating Cable Radiation Pattern RF in from transmitter RF out (terminated)
- 34. Panel Antennas
- 37. Traveling-wave Wire Antenna Reflected wave Dipole antenna Incident wave Resonant wave of wavelength antenna Reflected wave Incident wave Antenna Traveling wave for non-simple antenna
- 38. Folded Dipole Antenna Beam Driven element length = Reflector length 2 + 5% Radiation pattern Folded dipole antenna
- 39. Turnstile Antenna Turnstile antenna Radiation pattern
- 40. Loop Antenna Loop antenna Radiation pattern in horizontal plane
- 41. Rhombic Antenna L L L L 800 Preferred direction of radiation L L L L 800
- 42. Rhombic Antenna Radiation Pattern
- 43. Yagi-Uda Antenna Yagi-Uda Antenna Director Driven element Reflector
- 44. Yagi-Uda Antenna
- 47. Sector Antennas 3-sector cell 6-sector cell
- 48. Sector Antennas Realistic antenna coverage in 6-sector cell Antenna overlap in 6-sector cell
- 51. Space Diversity
- 54. Antenna Downtilt Beam of vertically- mounted antenna Beam of vertically-mounted- antenna with tilted beam
- 65. Multi-Beam Antennas Standard cell divided into 18 microsectors
- 66. Smart Antenna Systems Fixed Beam Strategy Adaptive Beam Strategy
- 68. Traffic Load Balancing Smart Antenna Systems Cell with unbalanced load Cell with balanced load
- 69. Handling Capacity of Smart Antenna Systems
- 71. Antenna Covers and Support Structures
Editor's Notes
- © Copyright 2001 Global Wireless Education Consortium All rights reserved. This module, comprising presentation slides with notes, exercises, projects and Instructor Guide, may not be duplicated in any way without the express written permission of the Global Wireless Education Consortium. The information contained herein is for the personal use of the reader and may not be incorporated in any commercial training materials or for-profit education programs, books, databases, or any kind of software without the written permission of the Global Wireless Education Consortium. Making copies of this module, or any portion, for any purpose other than your own, is a violation of United States copyright laws. Trademarked names appear throughout this module. All trademarked names have been used with the permission of their owners .
- Partial support for this curriculum material was provided by the National Science Foundation's Course, Curriculum, and Laboratory Improvement Program under grant DUE-9972380 and Advanced Technological Education Program under grant DUE‑9950039. GWEC EDUCATION PARTNERS: This material is subject to the legal License Agreement signed by your institution. Please refer to this License Agreement for restrictions of use.
- Antennas are part of all radio communications systems. Major uses include amplitude modulated (AM) and frequency modulated (FM) broadcast radio, broadcast television, amateur shortwave radio, public communications systems (fire, police, ambulance, etc.), military communications systems, radar, space exploration, satellite applications, and cellular telephony. The antenna is a critical part of a wireless system. It is the part that sends the downlink signal to the user and receives the user’s signal back on the uplink. The effectiveness with which the antenna functions impacts the effectiveness of the entire system. Most problems that users experience with a wireless system are due to the antenna system effectiveness and antenna strategy in the terrain in which it is used. In a wireless system, there are two antennas: The antenna at the base station The antenna on the mobile device (cell phone, etc.) Most of this module is devoted to the base station antenna because its effectiveness and efficiency impact all wireless users and define the quality level of the wireless system. The mobile device’s antenna is constantly on the move and subject to changing conditions, so not much optimization can be done for it. Its power must be kept low to avoid health problems for the user. (For more information, refer to the GWEC module HS-PHS: Public Health and Safety. ) In this module, you will learn how antennas transmit and receive signals, fundamental characteristics of antennas, types and features of antennas, and signal coverage problems and how to overcome them. You will also learn how to perform a return loss measurement and gain measurement for an antenna.
- After completing this module and all of its activities, you will be able to: Explain how an antenna transmits and receives signals. Explain fundamentals characteristics of antennas. Radiated power Antenna gain Beam width Front-back ratio Describe features of different types of antennas. Describe the different types of radiation patterns. Explain why and how to measure impedance. Explain strategies to address signal coverage problems. Explain antenna diversity and isolation strategies. Perform a return loss measurement on an antenna. Perform an antenna gain measurement.
- The antenna is one component of the wireless communications system. Technically, the antenna is referred to as the antenna system . This system handles both transmitting a signal to and receiving a signal from the mobile wireless device (the mobile station). The antenna system may consist of the following components: Transmit antenna Receive antenna Duplexer Multicoupler Combiner Isolator Tuning cavities Cabling The base station antenna is typically located on the top of the tower. However, when the tower accommodates multiple antenna systems, antennas may be located at various heights on the tower.
- The antenna itself is a series of metal wire, rods, or other shapes. It transmits when an electric signal of radio frequency is applied to it. The current generates an electromagnetic (EM) field around the antenna. This EM field moves outward from the antenna; it is sometimes called the radio beam or antenna beam . At the receiver, another antenna does the same thing in reverse. The receive antenna detects the EM waves that pass through it and cause an electric (AC) current to move in the antenna wire. This current is sent via the feedline to the receiver where it is decoded and amplified. An antenna transmits and/or receives radio waves through free space. The transmit antenna converts the power from the radio into a distinctly shaped radiation pattern. The receive antenna collects signals from the mobile station for input into the system. Antennas work because electric and magnetic fields propagate from a conductor that is carrying a varying electric current. If the frequency of the current is high enough, the strength of the EM field that propagates outward from this conductor will transfer signals from one point to another. These waves spread out like the ripples that occur when a stone is thrown into a body of water. EM waves travel at the speed of light. An antenna preferentially transmits frequencies whose wavelength is related to its physical dimensions. Often, the length of wire is naturally tuned to a particular radio wave length that is a simple fraction or multiple of that length. Thus, an antenna is designed with respect to wavelengths (λ) or fractions of wavelengths (λ/2, λ/4, etc.). The most common length for an antenna is one-half a wavelength, or λ/2; this is also known as the resonant length .
- The resonant length changes with the frequency and wavelength of the electric signal. The higher the frequency, the shorter the wavelength, and the shorter the required antenna. The lower the frequency, the longer the wavelength, and the longer the required antenna. The wavelength for cellular telephone transmission is approximately 0.33 meter. Therefore, the length of a cellular antenna should be 0.165 meter. Recommended cellular bands are as follows: Base station receive: 824-850 MHz Base station transmit: 869-896 MHz
- An antenna has the same radiation pattern and gain regardless of whether it is being used to transmit or to receive EM waves. It also has the same radiation pattern in both receive and transmit modes. When both a transmitter and a receiver are needed, their electronics are usually housed in a single box called a transceiver . Mobile and portable transceivers generally use a single antenna for both transmission and reception. For efficient operation, the coaxial cable that is the feedline from the transmitter to the antenna should be terminated with its own characteristic impedance. When this is done, the maximum power is passed to the antenna. If the terminating impedance does not match the impedance of the feedline, there will be reflections from the point of impedance discontinuity back along the feedline to the transmitter. This reduces the power passed to the antenna and causes the protection circuitry in the transmitter to reduce its output power, causing a further reduction in the transmitted power. Radio frequency (RF) transmission planning for the antenna system tries to optimize the signal strength received by the base station and mobile station regardless of their positions in the network. The type and configuration of the antenna system in the wireless system plays an important role in achieving this goal.
- Topics to be discussed include: Radiated power Antenna bandwidth Antenna gain Antenna beam width Antenna front-back ratio
- In the absence of reflection, refraction, diffraction, and absorption (in free space), the radio signal obeys an inverse distance law. The mean power received at any large distance is given by the Friis free-space equation (see above). The Friis free-space equation shows that the received power falls off with the square of the distance and it holds for large distances (several hundreds or thousands of meters).
- The range of frequencies in which the antenna radiates is called the antenna bandwidth . An antenna radiates to some extent over a very wide range of frequencies, down to an almost undetectable signal level. Therefore, antenna bandwidth must have some practical limit that relates to usable power radiation. Antenna bandwidth is defined to be the range of frequencies radiated where the lowest and highest frequencies have power that is 3 dB less than the power at the frequency with the maximum power, f max . The upper frequency, f up , is the frequency above f max where the power is 3 dB lower than f max . The lower frequency, f low , is the frequency below f max where the power is 3 dB lower than f max . Bandwidth can be expresses as a percent, B p , of the center frequency, f ctr . This form is normally used for narrowband antennas. Bandwidth also can be expressed as a ratio, B r . This form is normally used for broadband antennas. low up r f f B
- The antenna gain is the ratio of the antenna’s maximum radiation intensity to the maximum radiation intensity from a reference antenna with the same input power. If the reference antenna is an isotropic source of 100 percent efficiency, the resultant gain is called the gain with respect to an isotropic source. It is expressed in dBi. If the reference antenna is a simple dipole of typical efficiency, the resultant gain is called the gain with respect to a dipole . It is expressed in dBd. Antenna gain, G ant , is a function of wavelength. (See first equation above.) A e is the effective area of the antenna and is related to the physical or actual area of the antenna. (See second equation above.) ap is the effective length of the antenna, a measured quantity that varies from 0 to 1
- Since an antenna achieves gain by concentrating its radiation pattern in a certain direction, there is typically a simple relationship between antenna gain and beam width. The greater the gain, the narrower the beam width. The beam width is the width of the radiated pattern where the signal strength is one-half that of the maximum signal strength. At this point, the signal is 3 dB less than that of the maximum. The angle between the left and right points (as viewed from above) that are 3 dB down from the maximum is the beam angle or beam width . For unidirectional antennas, the resulting major lobe of the radiation pattern has a certain width. Common beam widths for cellular antennas are 60º, 90º, and 120º.
- The front-back ratio (commonly expressed as f-b ratio or f/b ratio) is the ratio of the radiated power in the intended direction compared to the radiated power in the opposite direction. In other words, the f/b ratio is a measure of the antenna’s ability to focus the radiated power in the intended direction successfully and not interfere with other antennas that are behind it. The f/b ratio is the radiated power on the main lobe of the antenna (arbitrarily placed at 0º) divided by the radiated power at 180º from that lobe. When gains are written as a number, usually in dB, the ratio of the two gains gives the f/b ratio. (See equation above.) P 0 = power radiated in the main lobe P 180 = power radiated in the back lobe This definition can create misleading results because there can be a null at 180º and a significant back lobe gain just away from 180º. Therefore, the f/b ratio should be interpreted as the gain on the main lobe minus the average gain over the rear 90º of the antenna centered at 180º.
- Antennas with good f/b ratio and unidirectional radiation patterns are what allow frequency reuse and more efficient use of spectrum because of better control determining where the actual radio coverage will be. This prevents interference with an adjacent site. Antenna f/b ratio is especially important in a cellular environment where the same frequency will be used in a nearby cell. The diagram above shows a generic cellular frequency reuse plan. Note that the same frequencies (represented by the same numerals in cells) are used in all directions repeatedly. The ability to radiate power in the desired direction is critical.
- Each antenna has a unique RF radiation pattern. This RF pattern can be represented graphically by plotting the received power by a test receiving antenna as a function of angle. The test receiving antenna is located far from the transmitting antenna. Cross-sectional slices are taken across the horizontal and vertical planes of the antenna's three dimensional pattern. These slices are then plotted on graphs. The radius of these graphs represents the expected RF power that will be radiated from the antenna. This power varies with the angle. The expected power gain at some point within the pattern is then predictable. Every subscriber's location will have a horizontal and vertical reference from the antenna. These horizontal and vertical values are positioned on their respective graphs and the results added/subtracted from each other. This value represents the theoretical gain of the antenna at that location. The pattern is representative of the antenna's performance in a test environment. A pattern is produced with distinctive major and minor lobes. This pattern only applies to the free-space environment in which the test measurement took place. Upon installation, the pattern becomes dynamic. The communication system's performance becomes more complex due to factors affecting propagation in the real world. These factors may include refraction, reflection, fading, etc. The real effectiveness of any antenna is measured in the field. There are three basic types of antenna radiation patterns: Isotropic Omnidirectional Unidirectional
- An isotropic antenna is a completely non-directional antenna that radiates and receives equally well in all directions. It is a theoretical point source or receiver. Its radiation pattern is spherical. Since all practical antennas exhibit some degree of directivity (preferential radiation in one direction), the isotropic antenna exists only as a mathematical concept. The isotropic antenna can be used as a reference to specify the gain of a practical antenna.
- The omnidirectional antenna is often simply called an omni antenna . It has a uniform radiation pattern in the horizontal direction. (See left graph above.) In the vertical direction, the radiation pattern is somewhat concentrated in the horizontal plane since that is where most of the users are located. (See right graph above.)
- As the name indicates, unidirectional antennas (also known as directional antennas ) radiate power mostly in one direction horizontally. This is done because the antenna is a passive device and the only way to obtain a gain in the desired direction is to concentrate the radiated power from another direction in the desired direction. Doing this is more effective and efficient than the brute force methods of omnidirectional antennas which simply increase transmitted power. A a reflector is often used to increase the gain of an antenna that does not radiate equally in all directions. The gain achieved is called antenna element gain . Gain is achieved in one direction at the expense of signal power in other directions. In a wireless cellular environment, this is desirable since the mobile station is in only one direction at a time. The patterns displayed in the graph on the left are those of a directional antenna (solid line) and an isotropic antenna (dashed line). The antenna's gain is approximately 15 dBi. Each graph ring equals 2 dB. The graph on the right shows the vertical radiation pattern of a typical unidirectional antenna. It is highly concentrated and provides additional antenna gain. The horizontal and vertical beam widths are found using the points at which the power is 3 dB down from the maximum. Alternatively referred to as half-power points , the 3 dB down points indicate the arc, in degrees, that the horizontal and vertical beam widths encompass. For the above antenna, it is 100º horizontal. In wireless telephony cells, radiation also is concentrated in the horizontal plane. This results in even more gain. Several unidirectional antennas can be stacked vertically to provide more gain.
- For a dipole and an isotropic antenna with the same input power, more power will be radiated by the dipole in the horizontal direction than by the isotropic antenna. The isotropic antenna will radiate more power in the other directions than will the dipole antenna. The difference in gain between the dipole and the isotropic antenna is 2.15 dB. The diagram above illustrates the differences in gain between the isotropic, dipole, and practical antenna. When choosing an antenna for a specific application, consult the manufacturer's data sheet. The data sheet contains such information as antenna gain, beam width (vertical and horizontal), and graphs of the vertical and horizontal patterns. Examples of the types of graphs normally found in a data sheet are shown in the diagrams on this and the previous two pages.
- Unidirectional antennas provide increased gain in a limited direction and multiply the use of separate channels by virtue of enabling sectorization. They do not overcome the major disadvantages of omnidirectional antennas but unidirectional antennas limit co-channel interference by receiving only from a third of the area around the site.
- Antenna polarization is an important part of a properly functioning system. With transmit and receive antennas at 90 degrees, the losses to the combined signal can approach –20 dB. Technically, this is an infinite reduction of more than –100 dB. An important property of a radio wave is its polarization. A radio wave, like all EM waves, has a magnetic field H and an electrical field E. The orientation of the electrical field determines the polarization. If the electrical field is vertical, the radio wave is polarized vertically. If the electrical field is horizontal, the radio wave is polarized horizontally. Theoretically, for a radio wave to be received at the maximum gain, the receive antenna should be oriented in the same direction as the polarization of the transmit antenna. In reality, while the base station antenna is usually mounted vertically, mobile antenna orientation can assume any direction. Fortunately, due to the advantageous effect of diffraction and reflection, enough signal can reach the mobile antenna to allow two-way communication in spite of unmatched mobile antenna orientations.
- When the interface between the transmission/coaxial (T/C) lines and the antenna connector is matched in impedance, all of the power is transferred and none is reflected. When there is a mismatch, a standing wave is produced. The ratio of maximum voltage to the minimum voltage of the standing wave along the transmission line is called voltage standing wave ratio (VSWR) . The closer VSWR is to the value of one, the greater the efficiency of electrical power transfer. VSWR is a measure of the impedance match between the antenna and the transmission line or coaxial cable. It is important to match the antenna to the T/C lines because a mismatch causes the affected element to reject part of the signal and send it back down the T/C lines. The rejected signal then reflects back and forth between the antenna and the transmitter. The percentage of the signal rejected depends upon the severity of the mismatch. This creates a fixed, measurable wave pattern along the T/C lines, which is quantified using the VSWR. The formula for VSWR is provided on the slide.
- The most commonly used antenna types for wireless telephony base stations include the following: Omnidirectional antennas Half-wave dipole antennas Hertz antennas Marconi antennas Radiating cable antennas Multi-antenna systems Panel antennas Unidirectional antennas
- The half-wave dipole antenna is the simplest form of the Hertz antenna. It is a straight conductor cut to one-half the electrical wavelength with the RF signal fed to the middle of the conductor. The diagrams above illustrate the radiation pattern of the half-wave dipole antenna, normally referred to as a dipole . Note that the radiation pattern has the shape of a doughnut (torus). A half-wave dipole antenna may also be used as a gain reference for practical antennas . In many instances, it is not desirable to have both sides of the dipole fully extended. In these cases, one of the sides, typically the lower side, is coiled up into a helix. This arrangement is sometimes called a monopole , and has almost the same properties as the simple dipole. Another arrangement, popular in cellular mobile stations, is to use the telephone body as the lower side of the dipole.
- An omnidirectional antenna is a simple dipole or monopole. (Also see discussion on Dipole Antennas in this module. ) An antenna having a physical length equal to one-half of the wavelength ( λ /2) is called a Hertz antenna , as shown above. Hertz antennas are used predominantly at frequencies above 2 MHz. The impedance of the Hertz antenna depends on where the feed line is attached. When it is attached at the center, as shown above, the antenna appears purely resistive and is 73 ohms. Feeding the same antenna at the end would change the impedance to approximately 2500 ohms.
- So called because it was the form of antenna used by Marconi, the Marconi antenna has a physical length equal to one-fourth of the wavelength ( λ /4). In addition, the transmitter-receiver must be grounded. The ground connection produces an “image” antenna of equal length. This makes the effective electrical length of the Marconi antenna equal to λ /2. Marconi antennas are used primarily at frequencies below 2 MHz. The impedance of this antenna with the signal applied at the base is approximately 36 ohms, or one-half the impedance of a Hertz antenna.
- The omnidirectional antenna radiates and receives equally well in all directions in the horizontal plane. This results in the signal power being spread uniformly in all horizontal directions, regardless of the location of the intended receiver (mobile station). Only a small percentage of the radiated power reaches the receiver. Because of this limitation, one strategy is to boost the power level of the signal being broadcast. Boosting the signal power can make matters worse for several reasons. The excess signal in directions other than that focused on the intended receiver can reach other receivers and result in interference for them. The excess signal in other directions inevitably is reflected off of one or several structures in the radiation area. These additional signals sometimes reach the receiver (intended and other). Since they have longer paths to travel, they arrive later and cause interference. This is called multi-path distortion . The reverse is true for an omnidirectional receiving antenna. It receives signals equally well from all directions in the horizontal plane and has no preferred direction. For a mobile transmitter to be distinguished, it must be stronger than the other signals as well as the background noise. Because of these limitations, omnidirectional antennas have very limited spectral efficiency and very limited reuse of frequencies in adjoining areas. The solution has been to abandon the use of omnidirectional antennas and use some form of unidirectional antenna.
- The slotted coaxial cable antenna is often referred to as a leaky cable or radiating coaxial cable. For transmission, slots in the corrugated copper outer casing allow a controlled portion of the transmitted power to escape along the entire length of the cable and radiate into space. The coverage obtained can be controlled both by the power of the transmitter and the pattern in which the cable is run. For reception, a signal transmitted near the cable will couple with the metallic element inside the cable through these slots and be carried along the cable to the receiver. The radiation pattern is a snaky cylinder that follows and surrounds the cable. As the signal is attenuated due to radiation and losses in the cable, the radiated power decreases. The radiating cable must be properly terminated at the RF-out end (far end) in order to minimize reflections. A leaky cable is a good application for an area of any shape (open or enclosed) that requires localized coverage such as: Along the ceiling of a large factory Along the perimeter of a large room Through certain cubicles in a large space In tunnels Wireless communications can be provided to locations that normally would have serious blockage or multipath interference. Because of its broadband capability, such a cable system can handle two or more communication systems at a time. Drawbacks to using a leaky cable are that it is quite expensive and no gain is achieved.
- A multi-antenna system contains a number of individual antennas that contribute to an overall radiation pattern. Examples of multi-antenna systems include the following. A pair of directional antennas mounted in opposite directions so that their radiation patterns point along a highway in either direction. This would cover a much larger, long, narrow area than would using two separate cell sites, and it would do so with fewer channels. A series of antennas around a building or construction site covering the entire site. This is used where an omnidirectional antenna would cause unacceptable levels of interference or where more gain is needed than an omnidirectional antenna can provide. A series of antennas located on the side of a single building covering the spaces at that end of the building. This arrangement enables lower power to be radiated in a more controlled space so that interference with other receivers is minimized. The multiple antennas are driven by a single transmitter, and the power is distributed by a power splitter. The typical gain of a multi-antenna system is the sum of the gains of the individual antennas minus the loss due to the power splitter. This type of antenna consists of two or more half-wave dipole antennas oriented vertically and stacked end-to-end. They are energized in phase. The radiation pattern that results is like the radiation pattern of the simple dipole, except flattened by about 50%. Since the radiation is concentrated vertically, the gain is also increased in the horizontal plane.
- Panel antennas are antennas that have a reflecting screen with simple radiating elements mounted over the screen. They are fully contained in a "low profile" box that is not as visually distracting as many other antennas. The beam of the panel antenna is concentrated by the use of the reflectors. Typical beam widths are 60º, 90º, and 120º. The antennas in the panel can be a dipole, co-linear array, or a parasitic array. An array may consist of one or more panels connected together. Panel antennas may use full-wavelength dipoles, half-wavelength dipoles, or slots as radiating elements. Advantages of the panel antenna include: More constant gain, radiation patterns, and low VSWR over a wide bandwidth Compact physical construction Very low coupling to the mounting structure Low side and rear lobes With proper design, each panel has a unidirectional pattern with the beam direction normal to the pattern. Panel azimuth beam widths can be adjusted to tune the signal coverage performance. Panel antennas do not need radomes (covers) to protect them from the elements or improve their visual appeal. They can be mounted directly on the sides of buildings with little visual impact. The dipoles are plates and have a wide bandwidth compared to a linear device.
- Unidirectional antennas are the form of antenna most used at cellular base stations. They focus their beams in one direction, which enables them to a chieve more gain and to r educe interference with other cell sections. Unidirectional antennas are also called beam or panel antennas because they concentrate the radiated power into a beam while minimizing emission in other directions. They can be classified into several categories: Linear Logarithmic Parasitic The beam width can be narrow or wide as needed by the application being served. The antenna gain depends on the degree of concentration of the emission relative to that of an isotropic or dipole antenna. Unidirectional antennas are broadband antennas, as compared to isotropic and Yagi antennas. This means that their propagation pattern and impedance does not change significantly over a wide frequency range. These antennas have a bandwidth ratio, B r , where: f up = upper frequency of range f low = lower frequency of range 2 low up f f B r
- Types of unidirectional antennas include the following: Traveling-wave wire antenna Folded dipole antenna Turnstile antenna Loop antenna Rhombic antenna Yagi-Uda antenna Log periodic antenna Mobile antenna Collinear gain antenna Subscriber unit antenna Sector antenna
- A traveling-wave wire antenna is so named because the wave traveling from the antenna connection point to the free end of the antenna is not a resonant wave but a traveling wave. Remember the condition set for the dipole antenna is the antenna by some simple multiple or fraction of the wavelength. This makes the dipole antenna resonant at those wavelengths. When the wave reflects off of the free end of the antenna, as shown in the diagram on the left, the reflected wave is in phase with the incident wave to create a resonant wave (also known as a standing wave ). However, when the antenna is a length that is not a simple multiple or fraction of a wavelength, the wave that is reflected is out of phase with the incident wave as shown in the diagram on the right. The sum of the two waves is a standing or resonant wave. When the antenna is terminated by a resistive load, there is only a single traveling wave from the source to termination. The equation for the current in the standing wave situation is :
- The folded dipole antenna is another variation of the standard dipole (Hertz) antenna design. Its primary difference is the feed point impedance, which is typically 280 ohms. This is a close match to the commonly available 300 ohm twin lead transmission cable. It is the interference pattern caused by the coupled reflector that shapes the antenna pattern.
- Another variation of the dipole antenna is the turnstile antenna , so called because it looks like a turnstile when viewed from above. The turnstile antenna consists of two dipole antennas set at right angles to each other. The entire antenna is in the horizontal plane. The diagram on the left shows an overhead view. The radiation pattern of the turnstile is a result of combining the antenna patterns of the two dipoles. Each produces the characteristic doughnut shape in the plane in which the dipole exists. From the top view, there is one doughnut in the horizontal plane, and one in the vertical plane. (Refer to the diagram on the right.) This pattern can be achieved only if the dipoles are 90° out of phase. The input impedance of the turnstile antenna is 36 ohms, half the input impedance of the dipole antenna. Since the two antennas are fed 90 ° out of phase, the pattern seems to “rotate” in the plane shown, making one revolution for each cycle so it gives a time average the circular pattern in this plane.
- The loop antenna is simply a square or round loop of antenna wire. There may be only a single loop or there may be several windings of the antenna wire. The loop antenna design is used most frequently as a receiver antenna because of its large physical aperture. The larger the physical area of the loop, the more efficiently the loop antenna captures a signal. However, the loop is usually small compared to wavelength. The radiation pattern for the loop antenna when it is in the horizontal plane is very similar to that of the dipole antenna. Refer to the diagram on the right. Many factors should be considered for the loop antenna besides just how efficiently the wave front is collected. One consideration is that a broad antenna pattern may not provide enough rejection of unwanted signals, thereby decreasing the signal-to-noise ratio. Another consideration is power-overload of the receiver front-end circuitry. This will cause non-linear distortion and generation of unwanted intermodulation products. The loop antenna can receive more interfering frequencies than other antennas due to wider bandwidth, and therefore has more power arriving at the receiver. It is a relatively broadband antenna.
- An example of a linear antenna having directional capabilities is the rhombic antenna. It is essentially two Vee-beam antennas connected back-to-back. Each segment produces its own radiation pattern, as shown in the diagram above. The angles of the segments are such that one of the lobes from each segment aligns with each other to create a stronger main lobe. The main lobe projects in a forward direction, but there are side lobes arising from the other lobes of each segment.
- The radiation pattern of a rhombic antenna shown above is for each arm and resulting pattern of the antenna. For the lobes to align and the frequency response to be correct, there is a strict relationship for the design: The characteristic input impedance is 600 to 800 ohms. If the termination is not provided, the antenna will be non-resonant. The appropriate termination will make the antenna resonant with the frequency determined by the length of and angle between the sides of the Vee.
- The Yagi-Uda antenna , often referred to as simply the Yagi antenna , is an example of a beam antenna. The Yagi antenna is the most widely used directional antenna. It has a bandwidth of 65 MHz in all cellular bands . The Yagi-Uda antenna is favored because of its: High gain Low weight Low wind drag Low cost In cellular telephony, the Yagi antenna is used for: Base station directional antenna Testing or searching for interfering signals Remote area coverage The Yagi antenna is a parasitic array of elements with one driven element and one or more reflectors and/or directors. The parasitic elements of the Yagi antenna are so named because they are parasitically excited (i. e., currents are induced) by mutual coupling from the driven element, which causes them to become radiators.
- Since the reflectors are slightly longer and the directors are slightly shorter than the driven element, the resonant frequencies of these elements are slightly lower or higher than the driven element. As the wave fronts interact, cancellation and reinforcement occur to shape the propagating wave that emanates from the driven element. The array should be considered a structure that creates a traveling wave. Exhaustive analytical and experimental investigation have gone into understanding the properties and mechanism of the Yagi-Uda array since its introduction into the United States in 1928. Several empirical models have been developed that give predictable results. Computerized computational models allow rapid optimization of element length and spacing with minimal experimental testing.
- The log periodic antenna looks somewhat like a Yagi-Uda antenna, but is different. In the log periodic antenna, all elements are driven by the transmitter. The elements are typically 180º out of phase with the next element. All elements are driven, but they are not all active at the same frequency. This antenna has a broad frequency response and operates on more than one frequency. The length of the elements and their phase relationship determine the active elements at the active frequency. Usually those elements that are a half wavelength long are active or radiating at a given frequency. In wireless communications applications, log periodic antennas are most commonly used as the active elements in panel arrays of antennas. In comparison to an array of simple dipole antennas, an array of log periodic antennas has a higher gain and narrower beam width while operating over a broader bandwidth. Log periodic antennas also have a higher f/b ratio than dipole antennas.
- Collinear gain antennas are relatively low gain antennas producing between 3 dB and 5 dB gain. There are two common types: Standard mount Through-the-glass The standard mount antenna is mounted through a hole drilled in the vehicle. The antenna is in a single piece with the active parts connected metallically, and therefore provides superior performance. The through-the-glass collinear gain antenna is used when customers do not want to drill a hole in their vehicle. Instead, the antenna is divided into two parts and each part is affixed to opposite sides of the vehicle glass with adhesive. A small metal disk on each side of the glass provides a capacitive connection to couple the RF signal through the glass. In construction, both types of collinear antennas consist of an upper and a lower portion separated by a phase matching coil. (Refer to diagram above.) The phase matching coil adjusts the polarities of the signal. The center of the radio beam is at the top of the phase matching coil, so care must be taken to mount collinear antennas such that the phase matching coil is above any part of the vehicle. The antenna in a subscriber unit is built into the unit and is generally ¼ λ long. This equates to 3.5 inches long for 800 MHz cellular operation and 1.75 inches long for personal communication system (PCS) operation.
- When directional antennas are used in a sectorized cellular system, they are called sector antennas . This is because their unidirectional radiation characteristic is used to enable a wireless cell to be divided into three pie-shaped wedges of 120º each. Alternatively, the cell can be divided into six pie-shaped wedges of 60º, each using antennas that are more narrowly focused.
- The unidirectional antenna used for each sector is not actually unidirectional but has a series of side lobes that spread into adjacent sectors. There is also a small back lobe projecting into the opposite sector. Additionally, there is some overlap of the antenna radiation patterns from cell to cell. The amount of overlap can be reduced by using slightly more focused antennas, but this results in poorer coverage for the left and right edges (from the location of the antenna) of the sector. The radiation pattern does not just drop off suddenly, as shown in the above diagrams. It falls off gradually and extends into the next cell, or even the cell beyond that when conditions are right.
- Important considerations for base station antennas include: Isolation Isolation between antennas, transmit (TX) to receive (RX), and TX to TX Diversity RX antenna separation, if space diversity is desired Interference Radiation patterns must not be distorted by obstacles or reflections near the antenna Descriptions in this section are valid for 850 MHz, 1900 MHz, and collocated 850/1900 MHz systems, except where otherwise specified by band. The following descriptions state the requirements to attain an isolation of 30 dB for a TX to TX relationship and a TX to RX relationship. Two reasons for antennas to be separated from each other and from others are to achieve space diversity and isolation. The horizontal separation distance needed to obtain sufficient space diversity between antennas is 4 to 6 meters for TDMA/136 800 MHz band and 2 to 3 meters for TDMA/136 1900 MHz band. Typical values of separation distances between antennas to obtain sufficient isolation (normally 24 dB) are 4 meters horizontally and 0.2 meter vertically. If space diversity is used, the effect is an increase in signal strength of 3 to 6 dB.
- The diagram above shows a traditional configuration with space diversity . (Generally one foot is required for each ten feet in height above the ground.) The horizontal space needed for the antennas is dependent on the required diversity separation. Vertical separation requires approximately five times the horizontal value in order to get the same diversity gain.
- A dual-polarized antenna is an antenna device with two arrays within the same physical unit. The two arrays can be designed and oriented in different ways as long as the two polarization planes have equal performance with respect to gain and radiation patterns. The two most common types are: Vertical-horizontal arrays. Arrays in ±45 degree slant orientation. The two arrays are connected to the respective RX branches in the base station. The two arrays can be used as combined TX/RX antennas. The number of antenna units is reduced compared with space diversity. The diversity gain obtained from polarization diversity is slightly less then the gain from space diversity. In the most critical environments, such as indoors and inside a car, the gain is almost as good as if space diversity were used. A dual polarized antenna offers very low correlation between the two received signals, but the power reception of each branch is slightly better with space diversity. This implies a small benefit for space diversity in noise-limited environments. For most applications, the difference is negligible. In interference-limited environments, the low correlation obtained by polarization diversity is advantageous. Due to slightly different propagation characteristics for different kinds of polarization, the downlink from a ±45 degree dual polarized antenna suffers from about 1.5 dB extra loss compared to two vertically polarized antennas. The loss can affect uplink or downlink depending upon what the antennas are used for, e.g., RX or TX.
- In order to avoid distortion due to intermodulation, the transmit and receive parts of the base station must be isolated. The following isolation values should be fulfilled: TX - RX isolation > 30 dB TX - TX isolation > 30 dB The horizontal physical separation for 30 dB isolation is 11.5 λ. Horizontal separation is usually ten feet at 800 MHz and six feet at 1900 MHz. The vertical separation requirement for antennas is at least 0.2 meter.
- When an antenna is mounted vertically, the maximum beam intensity will be at a point along a horizontal line starting at the center point of the antenna. Curvature of the earth does not really adversely affect systems at 800 or 1900 MHz because the effective propagation of most cells does not exceed two to three miles and the vertical beam width is 15 degrees on a 12 dB antenna. The antenna could be mounted closer to the ground in an attempt to lower the elevation of the beam center and therefore increase its useful range. Obstacles close to the antenna may create shadows in the beam and attenuate its power for receivers in the shadow area. These obstacles will also cause reflections and multipath distortion. Another solution is to tilt the beam electrically or physically downward toward the earth’s surface. This downtilt brings the distant beam closer to the earth’s surface. This reduces the distance that the antenna beam reaches. It also provides the area close to the antenna tower with a portion of the radiated effect, known as null fill . The most common downtilt is 6 to 10 degrees below horizontal. Downtilting is usually done with the major lobe of directional antennas. If coverage is needed near the distance limit of the beam or in a valley, downtilting can aim the maximum beam at that area. Downtilting may introduce new problems: B-lobe is uptilted and may cause interference with co-channel and adjacent channel sites. Side lobes may be created and cause interference. With severe downtilt, an unwanted gain reduction called a notch may result in the major lobe. This notch will reduce coverage at the cell border. Electrical downtilt will affect every lobe of the antenna to a certain degree.
- Reducing the antenna height by 50% will reduce the average received signal level by 6 dB. Moving the transmit antenna is preferable to moving the receive antenna. However, repositioning of both the transmit and receive in the same direction is desirable in order to maintain system balance.
- Problems that occur in the signal coverage from antennas include the following: Design problems Maintenance problems System maturation Site location and geometry may not be perfect Shadows in the pattern due to obstacles between the antenna and the receiver(s) can create dead spots in the coverage area Nulls in the pattern due to interference creating dead spots in the coverage area Intermodulation, co-channel, and adjacent channel interference problems
- Once the sources of interference are known, solutions involving the antenna can be implemented by : Reducing the antenna height. Sometimes, the cheapest way to solve coverage problems is to mount the antenna geographically and at a low level. This will decrease the cell radius and increase the opportunity for greater frequency reuse. However, when coverage is required in distant areas, a higher antenna position with more gain will be required. This increases interference with adjacent cells. Downtilting the antenna. Using a higher or lower gain antenna. Using an antenna with a wider or narrower horizontal or vertical beam width.
- Return loss (RL) is the power difference between the incident and reflected wave on the transmission line feeding the antenna. The RL coming from an antennas is measured as VSWR, which was discussed earlier in this module. Refer to formula above. A 3 dB return loss means that the reflected power is half the incident power. The ratio will always be less than or equal to one since the reflected power must be less than or equal to the incident power. Therefore, log ρ will always be negative. The minus sign in the formula ensures that the resulting return loss value is positive. A good match between source and receiver impedance will result in low reflection. Such a match will have a low return loss, e.g., 30 dB RL. For total reflection, RL = 0 dB. For a perfect termination, RL = ∞ (infinity) = reflection coefficient
- A multipath condition is one in which the signal received by a receiver has come from the transmitter over more than one path. This occurs when the signal has been reflected off one or more objects in such a way that both the direct and the reflected signals reach the receiver. The two paths must inevitably be different lengths. This causes the two signals to be in varying degrees out of phase. Even if the phases might be in sync, when the receiver is a mobile phone that situation will not exist for any appreciable amount of time. .
- The multipath condition causes a number of undesirable effects: Fading Phase cancellation Delay spread Co-channel interference Fading . The out-of-phase condition of the signals causes interference and a reduction in signal strength. This is called Rayleigh fading or fast fading. (Refer to diagram above.) The multipath condition usually results in many areas or pockets where the conflicting signals interfere sufficiently so that fading occurs. These pockets move through space as the phase relationships change over time. A mobile receiver passing through these pockets will experience fluctuations in signal strength, sometimes causing degraded signal quality. Phase Cancellation . The extreme case of fading due to phase interference is when the two phases are 180º out of phase and cancel each other. Although this results in total loss of signal when it happens, the condition lasts only for a short time. It causes problems rarely because wireless interfaces are resilient enough that a call can be maintained for a short time while there is no signal. In the case of phase cancellation on the control channel, call set-up will fail. Delay Spread . Delay spread is a condition in which the time of receipt of the delayed signal from a longer path is delayed sufficiently that a digital interface is corrupted. This can result in intersymbol interference , loss of framing, and other digital protocol conflicts. The effect of this condition is an increase in bit error rate and degradation in signal quality.
- Co-channel interference occurs when the same carrier frequency reaches a receiver from two separate transmitters, probably originating in two different cells. This is one of the primary causes of signal degradation in wireless telephony.
- Advanced antenna systems are a solution to the demand for increased numbers of users in a cell. These systems are used to replace omnidirectional transmission and sectored cell systems in high capacity situations. Even though advanced antenna systems are very expensive, they can increase cell coverage and capacity without building additional cell sites, which is more expensive. Examples of advanced system antennas include: Multi-beam antennas Smart antenna systems
- In multi-beam systems (also known as switched beam systems ), a normal (macro) sector is subdivided into pre-set micro-sectors using several directional antennas with a very narrow azimuth beam width. (Refer to diagram above.) The micro-sectors are fixed in beam azimuth (horizontal angle), beam width, and effective beam length. The antenna for each microsector is most sensitive in the center of its beam. Therefore, the antenna system is easily able to reject interfering signals from off-center mobile stations. However, signals from on-center mobile stations are more difficult to reject, especially if the interfering signal is stronger than the intended signal. The antennas are under a common control for the sector. As the mobile station travels through the sector, it moves from micro-sector to micro-sector. The control mechanism uses a weighting scheme based on phase differences to determine which receive antenna has the strongest signal from the mobile station. It then uses that pair of receive and transmit antennas. There is a handoff from micro-sector to micro-sector that is identical to the handoff from sector to sector.
- Also known as adaptive array and steerable antennas , smart antenna systems use electronically steerable antennas to improve performance. The most commonly used smart antenna is a phased array system that produces 12 radio beams. These systems can modify the direction of the main power lobe to optimize gain in the direction of the mobile station. Each antenna can be electronically steered through approximately 120º azimuth. Smart antenna systems form customized beam patterns to focus the center of the pattern’s main lobe on the mobile station. This also distorts the normal side lobes. Depending on the air interface being used, the steerable antenna may be used on the receive link or both the transmit and the receive links.
- Time division duplex (TDD) communication systems transmit and receive on the same frequency. In TDD systems, the uplink and downlink signal characteristics can be treated as related and the uplink characteristics used directly to control the antenna steering. Frequency division duplex (FDD) systems transmit and receive on separate frequencies, e.g., GSM. Uplink and downlink signal characteristics should be treated as independent entities. In order to steer the downlink antenna, further processing of the uplink signal characteristics is required. Steering of the beam is done with sophisticated signal-processing algorithms to select the desired signal among the multipath continuously and interfering signals in the environment. It then calculates the direction and distance to the receiver and modifies the transmit strategy to optimize the signal received by the mobile station. Because smart antenna system radio beams can be oriented toward a specific mobile station and away from others, the same frequency can be used elsewhere in the cell for other mobile stations. The amount of frequency reuse is greater than with standard six-sector cell system whereby the frequency is used only once. This makes the capacity of a smart antenna system greater than that of a standard cell system. Under the fixed beam strategy, the power of the radio beam (and gain of the receive antenna) must be strong enough to override interference satisfactorily, even when the mobile station is at the edge of the beam. Because these beams can be oriented toward a specific mobile station, power used for the beam does not need to be as high as in a cell using a fixed beam strategy.
- When a cell’s users are not evenly distributed among the sectors of the cell, the load on the antennas of a standard six-sector cell is unbalanced. With a smart antenna system, the electronics that control the antennas can change the distribution of the sectors to divide a busy sector into two sectors and combine low-usage sectors into a single sector.
- Multibeam and adaptive beam strategies can be combined with conventional sectorization in the same sector in order to provide thorough coverage. It is common to have the conventional strategy used near the transmitter where signal strength is strongest, the multibeam strategy used in the middle distance where the signal strength is sufficiently strong, and the adaptive strategy used in the more distant parts of the sector where the signal strength is weak. Different breakdowns of the sector can be used to adapt the combined strategy to low and high interference environments. In a low interference environment, the portion of the sector covered by the conventional sectorization can be increased. The portions covered by the multibeam and adaptive strategies can be diminished, or the adaptive strategy can be entirely eliminated. In a high interference environment, the portion of the sector covered by the adaptive strategy can be increased. The multibeam and conventional strategies can be diminished, or the conventional strategy entirely eliminated.
- Several factors should be considered when deciding between switched beam and adaptive array systems. Interference Suppression . Switched beam antennas suppress interference by virtue of their directivity. However, because they are fixed in orientation, the suppression direction may actually be the direction to the receiver, unless the receiver is near the center of the main lobe of the beam. Adaptive array antennas reject interference by virtue of accurately focusing of a very narrow beam. This beam is able to be accurately directed to wherever the receiver is located as it moves through the sector. Adaptive array antennas offer superior interference rejection. Range and Coverage . Switched beam systems can substantially increase base station range (from 20% to 200%) by using strongly directional, highly sideband-rejecting antennas. Adaptive arrays are able to cover a wider area more uniformly due to their ability to direct the beam to the receiver dynamically. Spatial Division Multiple Access (SDMA) uses advanced processing to identify and follow mobile receivers and dynamically steer the transmission signal to follow them as they move throughout the sector. This enables the wireless system to use the available frequencies where the customers are located, rather than to waste them by using a fixed spatial allocation system. SDMA essentially creates a sector for each receiver because it can dynamically direct the beam and allocate frequency or channel to follow the receiver. It does this while maximizing signal strength at the receiver and minimizing interference. SDMA uses multiple antennas to combine signals in space at the location of the receiver. This is much more effective than both beam switching and beam steering, which select the one best path from a single antenna to the receiver.
- Most antennas used in wireless telephony are covered for the following reasons: Protect the antenna elements from the weather. Make the antenna more aesthetically pleasing. The antenna cover (also referred to as radome ) is made of a material, usually plastic, that is transparent to the radio frequency being used. If the cover is painted, the paint must also be transparent to the radio frequency. Support for cell site antennas can be any of the following: Self-supporting towers Guyed towers Monopoles Any existing structure such as buildings, water towers, silos, billboards, etc.
- A self-supporting tower is a large, three-dimensional framework of galvanized girders. It has three or four vertical girders that are spread apart to provide stability. These three or four legs get closer together as the tower rises. Side girders crisscross to provide strength and wind resistance. The antennas may be placed at the top of the tower or at any level on the tower depending on transmission requirements. A guyed tower is also made of crisscrossing steel girders. However, it is a more slender structure that is not designed to support itself. It is held in place and erect by large guy wires placed in the ground at a distance to form approximately an appropriate angle to support tower structure properly. The guyed tower takes up a larger plot of land, but does not occupy the plot as completely as does the self-supporting tower. Also, because it is lighter, it is also less expensive. Guyed towers also can be as high as 400 feet (125 meters). Similar to the self-supported tower, antennas on the guyed tower may be placed at the top of the tower or at any level on the tower depending on transmission requirements.
- Monopoles are the most aesthetically pleasing. They are prefabricated and available in heights ranging from 50 to 150 feet (15 to 46 meters). A monopole is a tapering tubular pole made of structural steel pipe. It comes in sections that are fitted together to reach the desired height. It is a self-supporting structure firmly bolted to a concrete foundation that is buried in the ground. The main advantage of the monopole is that it requires only about 10 square feet (3 m 2 ) of land area and is more pleasing aesthetically. The disadvantage is that it cannot rise as high as the self-supporting or guyed towers. Antennas may be placed at the top of the pole or at any level on the pole depending on transmission requirements.
- There is an increasing need for aesthetically pleasing antenna support structures. Often people object to large steel towers located in their neighborhoods. Municipalities may require camouflaged towers before they will grant permission for an antenna to be located there. Antenna towers that look like different varieties of trees can be used in such situations. Locating the antenna on an existing structure can be both cost effective and a way to make the antenna less noticeable. In this case, the antenna owner leases the space and access rights from the structure’s owner. Examples of such structures include the following: Buildings Water towers Electric towers Light poles Highway signs Antennas can be placed at the top of a structure so that its elements have access to all horizontal directions. When this is not possible or is undesirable, the antenna must be placed on the side of the structure where it is blocked from 180º of horizontal coverage. In this case, antennas are usually placed on several sides of the structure in order to achieve 360º coverage. Some structures require special lighting and/or paint to meet Federal Aviation Administration (FAA) safety rules. Additionally, the Federal Communications Commission (FCC) specifies the amount of power allowed based on the terrain, frequencies used, and other radio uses in the vicinity. The FCC can impose hefty fines to offenders who exceed the allowed power level.